Method of manufacturing and calibrating a displacement measuring sensor

ABSTRACT

A tensioned helical wire for use as a displacement sensor. The sensor made of high strength piano wire is attached at its ends to two anchors which in turn are attached to or are a part of the object or objects to be tested. In the preferred embodiment the sensor has straight ends which are attached to two small anchor blocks. These blocks are then attached to the test object. The displacement range and sensitivity of the sensor can be controlled by the initial geometry selected. If an initial tension applied is small, the sensor has a very large response range that is about fifty times that of a straight wire sensor. If a large initial tension is applied, the helical shape approaches that of a straight sensor and has a displacement range only a few times that of a straight wire sensor. If the sensor has a large displacement range, the sensitivity is much less than that of a straight wire sensor and if the sensor has a small range the sensitivity is comparable to that of the straight wire sensor. When in place, a readout system can be used to pluck the wire sensor to determine its initial vibratory mode. Should a sensor length change occur, this can be interpreted as a displacement change by using calibration results obtained during the manufacture of the sensor or during the installation process. Subsequent readings made by the readout system can be used to measure on a continuing basis the resulting displacement behavior.

This is a division of application Ser. No. 790,013, filed Apr. 22, 1977,U.S. Pat. No. 4,170,897.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A vibratory wire displacement sensor of helical shape whose mode ofvibration can be read out to indicate the displacement behavior betweenthe anchored ends.

2. Description of Prior Art

The use of vibrating wire displacement gages per se is old as evidencedby such references as U.S. Pat. No. 3,675,474 to R. D. Browne. Myinvention employs a similar principle of operation but by using adifferent method of construction is able to provide a much moreversitile sensor than the straight wire sensor. A straight wire of hightensile strength can elongate about 0.35 percent of its initial lengthbefore breaking. This restricts its use to what is known as theinfinitesimal range in which displacements of the order of micro inchesper inch are to be measured. If displacements greater than 0.35 percentare to be measured some other method is needed. This is commonly donewith linear voltage differential transformers. These units are expensivewhen compared to the cost of a vibrating wire sensor. In addition, ifstraight wire sensors are used for small displacements and linearvoltage differential transformers are used for large displacements, tworeadout systems must be purchased. In addition, the particular range ofdisplacements is built into the linear voltage differential transformer.The helical wire sensor range and sensitivity can be established at thetime of the test by the user. Because of the extremely low cost of thehelical sensor it can compete commercially with any other displacementor strain sensor now on the market. The helical wire was tested with alow voltage plucking system owned by the United States Governmententitled "Vibrating Wire Readout Meter". The readout meter was inventedby William V. Bailey while working as a Creare Incorporated employee ona United States Bureau of Mines contract. This meter is disclosed inU.S. Pat. No. 3,889,525. The prior art stressed the need for thevibrating wire to be straight to avoid multiple readings in U.S. Pat.No. 3,411,347 to J. Wirth et al. In another U.S. Pat. No. 3,963,082 toMeier straight wire vibratory sensors are used for weighing purposes. Inthis patent a number of springs of helical shape are used to linearizethe straight vibrating wire response. The straight wire sensors and notthe springs are made to vibrate.

SUMMARY OF THE INVENTION

A vibratory helical wire sensor for measuring displacements. The sensorafter manufacture as a close helical coil is elongated to a selectedgeometry to establish a displacement sensitivity range. The coil iscalibrated to establish the vibratory mode vs the displacement behavior.The sensor is tensioned and the ends are attached to two points on thesame body or to two points one each on two different bodies. An initialvibratory mode is recorded and changes in mode with displacement aredetermined.

The principle objective of this invention is an improved vibrating wiresensor.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an unmounted vibratory wire displacement sensor withstraight ends.

FIG. 2 shows an unmounted vibratory wire displacement sensor withcircular ends.

FIG. 3 shows a side view of the sensor of FIG. 1 attached to unmountedanchor blocks.

FIG. 4 shows a top view of the sensor of FIG. 1 attached to unmountedanchor blocks.

FIG. 5 shows a side view of the FIG. 3 assembly mounted on the surfaceof a test object after the sensor is tensioned.

FIG. 6 shows two anchor blocks without sensor attached to the surface ofthe test object.

FIG. 7 shows a section normal to the surface of the test object in whichthe shape of the object itself provides the required anchor supports andrelief for the helical length of the sensor to vibrate withoutinterference.

FIG. 8 shows the top view of FIG. 1 sensor attached directly to thesurface of the test object in a slot cut into the object with an endmill to provide room for the sensor to vibrate freely without touchingthe test object except at the ends.

FIG. 9 shows the tensioned sensor of FIG. 1 attached to two separateobjects to measure the displacement between them.

FIG. 10 shows a section through pins of circular section placed in drillholes in the test object, the pins supporting a tensioned FIG. 2 sensor.

FIG. 11 shows the top view of the pin and sensor assembly of FIG. 10.

FIG. 12 shows the mounted tensioned sensor of FIG. 5 with wire pluckingand readout instrumentation representation.

FIG. 13 shows the top view of an anchor block with a small slot forreceiving the end 2 of the sensor of FIG. 1.

FIG. 14 shows the end view of the slotted anchor block of FIG. 13 withthe end of sensor of FIG. 1 in place.

FIG. 15 shows the side view of the FIG. 13 anchor block with open wiregroove.

FIG. 16 shows the FIG. 14 view after the wire groove is crimped shut tosecurely anchor the end of the FIG. 1. sensor.

FIG. 17 shows the top view of a gage block used to place anchor blocksthe required distance apart and to bond them to the surface of the testobject.

FIG. 18 shows the end view of the gage block of FIG. 17.

FIG. 19 shows the side view of the gage block of FIG. 17 holding anchorblocks.

FIG. 20 shows a method of manufacturing the sensor by winding a smalldiameter wire around a cylindrical mandrel to produce a helical coil.

FIG. 21 shows the helical coil of FIG. 20 being stretched by tension ordisplacement means to create the sensor of FIG. 1.

FIG. 22 shows the procedure used to calibrate the helical sensor beforeit is attached to anchor blocks or to the test object.

FIG. 23 shows the procedure used to calibrate the helical sensor afterit has been attached to anchor blocks but before the blocks are attachedto the test object.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows one embodiment of a helical vibratory wire displacementsensor. The sensor consists of a helical portion 1 and two straightcolinear ends 2. If the sensor is tensioned between two points ofsupport and made to vibrate in a transverse direction at its resonantfrequency, that frequency can be determined by instrumentation and usedto measure the distance between the points of support. If the distancechanges because the body to which it is attached deforms this can bemeasured. If the ends 2 are attached to two separate bodies the rigidbody displacement between the bodies can be measured. FIG. 2 shows asecond embodiment of a helical vibratory wire displacement sensor. Thesensor consists of a helical portion 1 and two circular ends 3. In thesensors of both FIG. 1 and FIG. 2 the coiled portion 1 may have from oneto many turns and the coiled portion 1 need not extend from one anchorto the other but may do so. One method of mounting sensors of FIG. 1 isto use the anchor blocks 4 of FIG. 3 and FIG. 4 to which the ends of thesensor 2 can be rigidly attached. Any method of attachment that resultsin a rigid connection to the anchor blocks can be used. The sensor ends2 could be attached to the anchor blocks 4 by using epoxy or otheradhesive, welding, soldering, clamping, or crimping. In FIG. 3 and FIG.4 the sensor ends 2 are attached to the blocks 4 before the blocks 4 areattached to the test object. In FIG. 5 the anchor blocks 4 have beendisplaced to produce a tensile stress in the sensor 1 before the blocks4 are attached at 6 to the surface of the test object 5. In FIG. 6 theanchor blocks 4 have been attached without sensor to the surface of thetest object 5 at the lower surface of the block at 6. In FIG. 7 the testobject is of such shape that the sensor ends 2 can be attached to theobject itself while allowing the vibratory part of the sensor 1 room tooperate without touching the object beneath at 8. This could be in theform of a groove cut into the test object. One groove shape that couldbe used is that of FIG. 8. This groove shape 9 has been successfullyused with a straight vibratory wire sensor and this use is described inU.S. Pat. No. 3,914,992. The groove shape 9 is conveniently cut with anend mill of 0.125 inch diameter. The ends of the sensor 2 can be bondedto the anchor sites 7 or can be crimped into two small colinear wiregrooves cut into the surface along the centerline of the slot 9 as inthe patent just cited. In FIG. 9 the sensor is attached to two separatebodies 11 and 12 which can displace relative to one another and thisdisplacement is to be measured. The displacement results in a change inthe sensor tensile stress which in turn results in a change in theresonant vibratory frequency of the sensor. In FIG. 10 two holes 13 havebeen drilled normal to the surface of the test object 5. Two cylindricalpins 15 with annular grooves 14 to hold the ends 3 of the FIG. 2 sensorare mounted in these holes so that they are rigidly attached to the testobject. The attachment can be by friction, epoxy or other adhesive, or athreaded pin and tapped hole. The pins are placed at some selecteddistance apart so that the FIG. 2 sensor is appropriately tensioned whenthe ends 3 are placed in the annular grooves of these pins. The circularends 3 may be bonded with epoxy to the pins so as to establish avibratory length that corresponds to the spacing between the pins. InFIG. 11, the top view of FIG. 10, the surface of the test object 5, theends of the pins 15 and the FIG. 2 sensor are shown. FIG. 12 shows thelocation of a wire plucking magnet connected to a readout instrumentshown symbolically as 16. One anchor system proven to work well isdescribed in U.S. Pat. No. 3,914,992 where the slots are cut into thebody itself and these slots are then crimped shut to anchor the ends ofa straight wire sensor. Anchor blocks with wire anchor slots are shownin FIGS. 13, 14, 15, and 16. The slots in the blocks 17 in these figuresare denoted by 18. The wire used in tests to date to make the sensors ofFIG. 1 has been of 0.009 inch diameter 302 stainless or conventionalpiano wire. The wire anchor slot 18 has been 0.010 inch wide and 0.028inch deep. The ends 2 of the sensor of FIG. 1 are placed in the bottomof the slot 18 and the slot is forced to flow shut plastically by meansof a wedge shaped tool with a blunt end that is 0.050 inch wide. Thesensor ends 2 are then surrounded by anchor block metal holding thesensor end 2 securely as in FIG. 16. The holding force on the wire canbe calculated by the information given in U.S. Pat. No. 3,914,992. Inmany applications for displacement measurements a standard gage lengthis used. This can be accomplished by the gage block 19 of FIG. 17. Thisblock has two openings of rectangular shape 20 into which the anchorblocks 4 are placed. This block may be magnetic so that if the blocks 4are made of steel they will remain in place in the gage block 19. Theblock 19 has a selected gage length called G.L. which spaces the blocks4 the required distance apart. If adhesive material such as acyanoacrylate cement or epoxy is spread over the lower surface of theblocks 4 and then this surface is pressed against the surface of thetest object 5 the region 6 in FIG. 5 will become rigidly attached to thetest object when the adjesive hardens. The gage block 19 is removed andreused. Gage blocks of several different lengths can also be used tocalibrate the sensor response.

The vibratory wire helical displacement sensors of FIG. 1 aremanufactured in the following manner as shown in FIG. 20. Piano wire 23of 0.009 inch diameter is wound into a helical coil 21 about acylindrical mandrel 22 of selected diameter. In tests to date a range ofsizes from 0.020 inch to 0.050 inch has been tried with this wire size.A diameter of 0.025 inch has worked well although other sizes are alsoall right. In FIG. 20 the wire diameter is denoted by d and the mandreldiameter by D. By changing d and D a wide variety of coil geometries andflexibilities is possible. The ends of the coil 23 are attached to means24 for stretching the coil and changing its geometry, as shown in FIG.21. A straight wire sensor has a displacement range of about 0.0024 inchin a 0.75 inch gage length corresponding to a vibrational frequencyrange of 2000 to 6667 cycles per second. This is a 0.32 percent changein the gage length. By contrast, a variety of helical coil displacementsensors tested had a displacement range of 1 to 15 percent or more ofthe gage length. This displacement range was established by themagnitude of the force applied to elongate and to establish the initialgeometry of the sensor of FIG. 21. If a force is applied that is nearlythe breaking strength of the wire, a displacement of about 1 percent ofthe gage length results. Even under these stress conditions the helicalsensor retains some of its helical shape. If ten or twelve turns areused and a small tensile force is applied the sensor has a finite rangedisplacement of perhaps 15 percent. The flexibility of the helicalsensor is defined to be the relative displacement for a given gagelength and applied tensile force compared to the displacement for astraight vibratory wire for the same conditions. Flexibilities in therange 3.4 to 1015 were observed in tests. That is, the helical sensorselongated 3.4 to 1015 times as much as the straight wire sensor for thesame gage lengths and tensile forces applied. The flexible helicalsensor has a range and corresponding vibratory frequency that needs tobe stablished by a calibration process. In FIG. 22 the calibrationprocedure for the FIG. 1 sensor which is unmounted is described. Theinitial gage length is called G.L. 1. The sensor is stretched by a forceor displacement means to a second gage length G.L. 2 which is thelargest value that may be used and is not to be exceeded in use. This isrequired because it establishes the elastic range of the sensor. Thelarger the tensile force or displacement applied the more nearlystraight the sensor and the smaller the displacement range. If the G.L.2 is small, the sensor remains very flexible and the displacement rangewithin the elastic limit of the wire material is very large. Thesensitivity of the sensor is related to the displacement range. As thedisplacement range decreases the sensitivity increases and approachesthat of a straight wire sensor just before the sensor is broken intension. On the other hand if the displacement range is large thesensitivity decreases. It becomes necessary therefore to select the wiregeometry before use to determine what the best combination ofdisplacement and sensitivity will be. The meter reading is taken for theG.L. 2 chosen. The tensile force in the sensor is reduced to allow thesensor to shorten to a third gage length G.L. 3 where a second meterreading is taken. A fourth gage length G.L. 4 is chosen between the gagelengths G.L. 2 and G.L. 3. the sensor is attached with this gage lengthto the anchor blocks 4, to the anchor blocks 17, or directly to the testobject as in FIG. 7 or to test objects as in FIG. 9. While the sensorends shown in FIG. 22 are straight as in FIG. 1, item 2, the sensor ofFIG. 2 could be used in essentially the same way and could be anchoredadditionally by pins 15 in FIG. 10. The calibration described in FIG. 22could be done by the manufacturer before sale to customers or could becalibrated by the customer. This calibration could be performed by adisplacement tool not described that has vernier or micrometerdisplacement means to measure the gage lengths to at least 0.001 inchand more accurately if necessary. This could be done by attaching thesensor ends 2 or 3 to the jaws of a vernier caliper with a screw fineadjustment feed. The sensor ends 2 could be held by clamping means thatsecure the ends 2 only during the calibration process. The sensor isthen tensioned and attached to two gage blocks as in FIG. 6, the tensionbeing adjusted by force or displacement means while the wire is tuned togive the initial reading desired. In this way a desired G.L. andvibratory mode can be chosen. Another calibration procedure isillustrated in FIG. 23. In this figure the sensor is attached to gageblocks 4 before the blocks are attached to the test object. The sameprocedure is followed as for FIG. 22 but in this case the sensor withG.L. 4 is mounted directly to the test object as in FIG. 5. In this casethe gage length G.L. 4 is not specified in advance and for manyapplications this approach will be preferred since any gage length canbe chosen as long as it is known. In the FIG. 23 calibration approachthe blocks 4 are detachably mounted to a displacement or force means toprovide the four gage lengths shown. In this figure G.L. 1 is selectedduring the manufacture of the sensor. The G.L. 2 has a greaterdisplacement than G.L. 1, G.L. 3 or G.L. 4 and a vibratory frequency orperiod is determined. The G.L. 3 is chosen so that the displacement isless than G.L. 2 and enough different than G.L. 2 that the vibrationalmode of these two lengths can accurately define the sensor behavior vs.displacement.

The displacement of a straight tensioned vibratory steel wire sensor isdefined by the equation

    δ.sub.1 =4L.sub.o.sup.3 Pf.sup.2 /Eg=9775L.sub.o.sup.3 (1/T.sub.1.sup.2)

where δ₁ is the displacement between the ends of the sensor, inches;L_(o) is the initial gage length between the anchored ends, inches; f isthe resonant vibrational frequency, hertz; E is the Young's modulus30×10⁶ psi; g is the acceleration of gravity of 386 inches per secondsquared; p is the average density of the sensor per unit length, 0.283pounds/inch³ ; and T₁ is the Creare vibratory wire meter reading.

The helical wire sensor has greater flexibility than the straight wiresensor so the above equation must be multipled by a correction factor KIn addition to the K correction for flexibility, a correction for lengthis also necessary since this becomes important when the diplacement isnot an infinitesimal as for the straight wire sensor.

The required equation for the helical sensor becomes

    (L.sub.o -L)L.sup.3 =KL.sub.o.sup.6 P(1/T.sub.5.sup.2 -1/T.sub.4.sup.4)/Eg.

The meter reading T₅ ² and T₄ ² correspond to an initial gage lengthL_(o) and a second gage length L. In initially deriving the K correctionfactor, the lengths L_(o) and L are known and solving the equationproduces K. The resulting correction factor K can then be subsequentlyused in the equation for determining the variable gage length, which isthe only unknown in the equation.

That the equation is correct is evident from the fact that in all thetests to date with a flexibility of 500 or less the average coefficientof determination has been about 0.9985. This means that the equation forL is linear within the experimental error in the tests. That is, theestimated gage length is a linear function of the true length. There maybe times when an experimental calibration of the sensor is necessary ordesirable.

I claim:
 1. A method for manufacturing and calibrating a helical wiresensor, which comprises the steps of:(a) winding a small diameter highstrength wire into a helical coil; (b) applying a first elongation tothe coil to form a helical sensor of a selected geometry, with a givenflexibility, elastic displacement range, and sensitivity which isinversely proportional to the displacement range; (c) determining avibratory more for the elongated coil having such selected geometry; (d)applying a second elongation to the coil which is less than the first,and obtaining a second vibratory mode determination; and (e) comparingsaid first and second vibratory modes to corresponding displacements tocalibrate the helical wire sensor prior to use.